1. Field of the Invention
The present invention relates to a method for managing walsh code channels allotted from a base station in a mobile communication system based on a code division multiple access (CDMA) system, and more particularly, to a method for managing walsh code channels in which excellent walsh codes are allotted with priority from a base station so as to improve quality of service.
2. Background of the Related Art
Generally, a mobile communication system based on a CDMA system spreads frequency width of a channel and gives different codes to users through this spread channel. Accordingly, the mobile communication system has an advantage that a plurality of subscribers can simultaneously use the communication system through one channel. Such a mobile communication system based on a CDMA system increases frequency capacity by 10 to 20 times as compared with a related art analog communication system and has a relatively high frequency efficiency. Furthermore, the mobile communication system can facilitate data communication and has a relatively better security as compared with other communication systems.
FIG. 1 is a block diagram illustrating a related art mobile communication system based on a CDMA system.
Referring to FIG. 1, a plurality of PCS/cellular terminals MT1-MTn for mobile communication are respectively connected to their base stations BTS1-BTS32 through a radio communication link. The respective base stations BTS1-BTS32 are respectively connected to a mobile switching center MSC1 through their base station controllers BSC1-BSC4. At this time, home location register/visitor location register (HLR/VLR) which manages moving location of the mobile terminals MT1-MTn is connected to the mobile switching center MSC1.
Origination operation of the respective mobile terminals in the aforementioned mobile communication system will be described below.
If the mobile terminal MT1 transmits an origination message to the base station BTS1, the base station BTS1 detects the origination message and reports contents of the origination message to the base station controller BSC1.
If it is determined that a frame-offset from the base station controller BSC1 is appropriate to be allotted to the base station BTS1, the base station controller BSC1 allots the frame-offset to the base station BSC1. If not, the base station controller BSC1 proceeds with frame-offset negotiation over maximum three times.
Subsequently, if the frame-offset negotiation has been successfully proceeded, the base station BTS1 allots a traffic channel for communication. Then, the base station allots a walsh code channel.
Subsequently, null data are transmitted from the base station BTS1 to the mobile terminal MT1.
The frequency allocation (FA), walsh codes, and the frame-offset, which are allotted from the base station BTS1, are transmitted to the mobile terminal MT1 through a paging channel. Accordingly, the base station BTS1 can temporarily synchronize with the base station controller BSC1 when the null data are transferred from the mobile terminal MT1. If the base station BSC1 synchronizes with the mobile terminal MT1, ring back tone from the switching center MSC1 is transferred to the mobile terminal MT1. Thus, origination sound occurs in the mobile terminal MT1 so that the origination call access procedures are finished.
Walsh code allocation technology for dividing channels in the mobile terminal MT1 which requests call access or handoff, at the base station BTS1 and spreading transmission data will be described in more detail.
In a CDMA system by Qualcomm in the United States, there are provided 64 different walsh codes which are respectively 64 bits. There are provided 64 channels W0,W1,W2,W3, . . . , W63 in the walsh codes. The channel W0 is used as a pilot channel which catches the location of channels and controls power when initiating the mobile terminal MT1. The channels W1-W7 are used as paging channels which inform the mobile terminal MT1 of a paging signal. The channel W32 is used as a synchronization channel which synchronizes the mobile terminal MT1 with the base station BTS1. The other 55 channels are used as traffic channels, respectively.
According to the communication system based on the CDMA system, if each mobile terminal MT1 requests call access or handoff, a channel manager provided in the base station BTS1 allots walsh codes. The channel manager allows walsh codes of 55 traffic channels W8-W31, W33-W63 to be on standby using a FIFO type queue and allots walsh codes to corresponding mobile terminals, sequentially, as occasion demands. At this time, 55 traffic channels W8-W31, W33-W63 turns on the base station BTS1 so that various processor boards load corresponding software(S/W) and are initiated when a task of each software is driven. In other words, a queue is initiated when software to the channel manager is normally operated. Thus, if a particular mobile terminal requests call access or handoff from the base station, the channel manager determines whether or not there are available walsh codes in the walsh codes which are be on standby in the queue. Subsequently, if there exist available walsh codes, the channel manager allots the walsh codes sequentially and performs call access procedures. If there do not exist available walsh codes, the channel manager informs the mobile terminal of the status of the current communication service, which is not available.
The walsh code allocation method is performed in such a manner that the walsh codes for 55 traffic channels which are sequentially initiated are sequentially allotted in the order of the first walsh code which is not allotted. In this regard, the walsh code for the traffic channel W8 is first allotted. However, if a certain time passes, it is not easy to recognize what walsh codes are allotted ahead.
As aforementioned, if the base station allots the walsh codes sequentially, the walsh code having poor peak factor characteristic may be allotted, which occurs due to unique periodicity of the walsh code. The peak factor is a parameter that expresses peak power in association with average power. If the peak factor characteristic is good, load exerted on a base station amplifier 20 shown in FIG. 2 is reduced. For example, if any system requires average power of 10 W that satisfies signal to noise ratio (SNR) and if the peak factor is expressed as 10:1, the base station amplifier should be designed to cover 100 W.
In that case, quality of service which is not relatively good is provided to the mobile terminal to which the walsh code having poor peak factor characteristic is allotted. In other words, if the peak factor characteristic is not good, large load is instaneously applied to an input terminal of the amplifier 20. For this reason, a parameter such as adjacent channel power ratio (ACPR) increases, and the range for securing linearity of an output signal is exceeded at an output terminal of the amplifier, thereby causing distortion of CDMA signal. As a result, interference between adjacent channels increases and thus it is difficult to achieve optimal quality of service.
Accordingly, the present invention is directed to a method for managing walsh code channels in a mobile communication system, that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for managing walsh code channels in a mobile communication system, in which a base station manages peak factor characteristic of each walsh code through one queue or two queues so that the walsh code having excellent peak factor characteristic can be allotted ahead.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for managing walsh code channels in a mobile communication system includes the steps of initiating a plurality of walsh codes for the unit of sector and frequency at a base station, dividing the walsh codes allotted from the base station to a terminal in response to peak factor characteristic to put excellent walsh codes in a main queue and poor walsh codes in a sub queue, allotting with priority walsh codes which are on standby in the main queue at the base station if the terminal requests allocation of the walsh codes, and allotting walsh codes which are on standby in the sub queue if the allotted walsh codes are null or if there are no walsh codes having excellent peak factor characteristic.
In another aspect, a method for managing walsh code channels in a mobile communication system includes the steps of identifying walsh codes having excellent peak factor characteristic and walsh codes having poor peak factor characteristic among a plurality of walsh codes allotted from a base station to a terminal to be on standby in one queue in set order, allotting walsh codes having excellent peak factor characteristic at the base station if allocation of walsh codes is required, and determining whether or not to allot walsh codes in response to peak factor characteristic of the next ordered walsh codes.
In the present invention, the base station can allot with priority walsh codes having excellent peak factor characteristic. Accordingly, it is possible to minimize interference exerted on adjacent channels during communication, thereby improving quality of service in the overall communication system.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.